The present invention relates to noise cancellation and, more particularly, it concerns a method and apparatus by which passive noise absorption and active noise attenuation with sound energy are combined to cancel noise in a flow of gases. Although the invention has a broad range of applications, it is particularly suited to cancellation of noise developed by jet engines, during ground testing, and will be described in that application.
Jet engines, particularly the older types which have a low air bypass, are extremely noisy devices and create an environmental problem. The noise, which originates mainly in the gas jet outside of the engine housing, is very difficult to control and meaningful solutions have not been found. A large amount of the energy is in the low frequency band (about 80 Hz) which is very difficult to attenuate. Sound at these frequencies can propagate for large distances if the atmospheric conditions are favorable and the intensity of these low frequencies is enough to rattle windows and plates in a range of up to a few miles.
Although some design modifications in Jet engines have produced noise reductions for aircraft during takeoff and landing, large radiated noise levels are still generated. This is particularly a problem during ground run-up testing when the engines may be running at full power for periods in excess of 30 minutes. For people working close to the plane, this can produce health problems such as hearing loss if proper precautions are not taken. For people living within the sound footprint of the jet noise, this can cause stress-related problems and affect property values. For the owners of the Jet testing facility, revenues could be reduced due to the restricted times when they are allowed to test the jet engines.
Considerable efforts are being made to quiet aircraft Jet engines during flight. Newer jet engine designs use a fan to blow colder air around the hot jet. Mixing occurs and produces a cooler, slower moving jet which radiates less noise, especially in the lower frequencies. The same principles are used in the design of "hush kits" which are retrofitted to existing noisy engines. Although the hush kits enable the planes to meet the in-flight requirements, the reductions are small and are not sufficient to eliminate the problem of ground run-up testing which requires quieting over and above that provided by the hush kits.
Currently, three approaches are being taken to minimize the effects of the noise generated by ground testing of jet engines. First, by choosing a time of day, a location, and an aircraft orientation for the engine testing, the annoyance to the surrounding community can be minimized. Sometimes, a man-made barrier is placed between the plane and a community to help reduce the noise propagation in a particular direction. Second, portable devices called "noise suppressors" are wheeled up to the back of the jet which exhausts into and through them. To aid in the noise reduction, the end of a pipe through which jet exhaust gases are passed, may be curved to direct non-absorbed acoustic energy towards the sky. The third approach is known as a "hush house," that is, a complete building with acoustically treated walls, intakes and exhausts. When the engines of an aircraft are to be tested, the aircraft is taken into the hush house and the doors are closed to completely contain the sound.
None of the present approaches to jet engine testing noise reduction meets a required combination of criteria, including good noise reduction, mobility and adaptability to different planes at different places on an airport, and affordability. To restrict testing to some period during the day, even with barriers constructed to shield some areas, results in no reduction of radiated energy; it is only redistributed. Moreover, under some environmental conditions, for example, when the ground is cooler than the air, more distant communities can experience high noise levels, particularly at the low frequencies.
Several schemes have been tried for noise suppressors. Generally, such systems rely on passive absorption to reduce the sound level, but the problem remains at the low frequencies which are difficult to attenuate. Where remaining sound is directed to the sky, some effectiveness occurs at high frequencies, but at low frequencies, the wavelength is comparable to, or greater than, the dimensions of the exit and good directivity is unobtainable. Another quieting method uses large amounts of water sprayed into the jet. Some reductions in the radiated noise take place, but the huge amounts of water needed make this impractical for most facilities.
The "hush house" is the one approach that does reduce the noise to completely acceptable levels. These purposely designed buildings, however, require much maintenance and repair to keep them in good working order. The cost of such buildings can be prohibitive for all but the most affluent airports.
It is apparent that the problems associated with noise generated during testing of aircraft jet engines represents a challenging need for effective noise cancelling technology. The technology is also needed in various types of machinery, such as gas and steam turbine generators, large compressors, gas operated tools and machines, pumps and the like, where noise is carried in a directional flow of gases.